Aircraft operating at altitudes above approximately 10,000 ft. are conventionally pressurized to avoid any need for supplemental oxygen for crew and passengers. Typically, the cabin pressure is maintained constant at about the atmospheric pressure typically found at an altitude of 10,000 ft. Moreover, the cabin air must be changed constantly, typically at the rate of 1 lb. per minute per passenger and crew member for passenger and crew comfort and well-being.
In the usual case, the air for pressurizing the cabin is drawn as bleed air from the main propulsion engines of the aircraft. It is then conditioned for temperature and humidity and distributed within the cabin. Ultimately, the air is discarded overboard through a cabin pressure vent/control valve.
The bleed air for cabin pressurization drawn from the main engines increases the fuel consumption of the main engines. The increased fuel burn is not insubstantial and as a consequence, poses constraints on aircraft range, particularly for large aircraft cruising at high altitudes for long periods of time and increases operating costs.
The current practice of discarding cabin air overboard from cabin pressure to ambient static pressure in such a way as to contribute to the thrust of the main engines is simple. However, it is an inefficient way to recover the energy in the cabin airstream. Needless to say, an improvement in energy recovery from cabin air provides an opportunity for fuel cost savings and/or an extension of aircraft range. Moreover, the ducting required to convey bleed air from the main engines not only adds weight to the airframe, but contributes somewhat to the complexity thereof and accordingly, to the capital cost of the aircraft. If such ducting can be eliminated, lower capital costs and lower fuel costs can be obtained. At the same time, it remains necessary to provide for cabin pressurization and cabin air change. To obtain the efficiencies of lesser capital costs and lower fuel costs, a more efficient system of cabin pressurization is highly desirable.